The present invention relates generally to vibrating equipment used for material processing, such as vibrator screen units used for separating or classifying particulate feed material as to size, and more particularly to a system for facilitating the designing, building and maintenance of such screen units by monitoring operational stress forces.
Vibrator screen units are well known for separating particulate feed material into various size classes. Such units include a pair of separated, generally vertical sidewalls or plates which support at least one and preferably several transversely positioned decks of apertured screening material. When multiple screening decks are provided, the upper screen materials have larger openings than those below. Upon generation of a generally vertical vibrating motion, particles fed to the decks are caused to bounce so that smaller-sized particles fall through the openings in the screen material, and larger-sized particles remain upon the deck. Using multiple decks, operators are able to generate a product of classified material in several size ranges.
Such screen units are designed with a specified amplitude and velocity, which is a function of the configuration of the plates, the size and type of the vibration generating device, the orientation of the plates and/or the screen decks, and fabrication and assembly techniques, among other factors well known in the art of designing and manufacturing such units. As a result, screen units of a particular model typically develop a fairly predictable system frequency upon operation, with individual units of a particular design developing small variations in system frequency from the model/design parameters. Over time, the system frequency of an individual unit often changes, influencing longevity and productivity. Designers attempt to design vibrating units so that they operate as far from the system frequency as practical while still maintaining desired production rates.
In the designing of such units, it is desired that the plates move synchronously, so that the screen decks are vibrated uniformly across the length of the deck. This synchrony not only enhances the classifying performance of the unit, it also prolongs operational life of the screen unit. When plates are not in synch, operationally-generated forces cause stress, among other places to attachment joints between the screen decks and the plates, and between the plates and laterally projecting spring mounts. Imbalanced units have shorter operational lives, require more frequent maintenance, and/or have less efficient classification performance.
Conventional vibrating screen units are provided with plates made of steel in the range of 0.3125 to 0.75 inch thick, which is strong in the axial direction. However, the plates are relatively thin in view of the production loads and work performed, and are susceptible to racking or twisting forces along the z-axis. Potentially damaging operational forces in vibrator screen units are caused, among other factors, by uneven or misaligned springs, uneven foundation mounts, improper vibrating speed, improperly installed screen decks, worn bushings and/or imbalanced flywheels on the vibration generator. Due to the wide variety of potential causes for vibrator unit malfunction, it is difficult for the average operator to detect when a unit is not operating according to its design parameters. It is even more difficult for the average operator to accurately diagnose the cause of the malfunction.
Conventional techniques for monitoring plate movement include the fastening of paper throw cards to the plates at designated locations, typically near the inlet and discharge ends of the unit, and near the vibration generator. Ideally the cards are mounted at corresponding locations on each plate at a corresponding end of the unit. However, due to the harsh operational environment of the vibrator unit (quarry, mine, gravel plant, road building site, etc.) and the variations in operator training, very often the cards on each plate at a designated machine are not properly placed for accurate results. An individual applies a pencil or similar marking instrument while attempting to hold his hands steady against the card while the unit is operating, and a pattern is generated by the unit, which varies by the style of unit involved. Typical patterns include ellipses, straight lines and circles. Upon drawing at least one trace or curve, or preferably a series of traces at one monitoring point, the user then moves to a corresponding card at another monitoring point on the machine and produces another trace or set of traces. Next, the traces of the respective plates at the same location are visually compared as to their two-dimensional (x and y-axes) similarity. If the patterns are angularly skewed, show blurred lines or are vertically or horizontally displaced beyond a designated range, the unit is judged to be out of synch, requiring modification of the plates or screen deck fasteners, change of speed of the vibration generating device or the device itself, or other modifications known to skilled practitioners to bring the traced patterns within acceptable degrees of similarity.
While the use of throw cards is the accepted technique for monitoring the operation of vibrator units, a significant drawback of this technique is that it is subjective, one cause being that the pressure applied by individuals varies, influencing the results. Some operators are anxious about standing next to the unit vibrating in the range of 900 rpm. As a result, the pressure applied by the user may vary, as well as the angle of the pencil to the throw card. Further, if the operator's hand moves while marking on the card, or if he shifts his weight or moves his feet, the results will vary. Such variation may apply on a card-to-card basis by a single operator, due to fatigue or subtle variations in stance or pressure at various points on the machine, and such variations increase when operator-to-operator technique is compared. On large-sized vibrator units, some points on the machine are too high to reach when standing, and due to instability, ladders are not placed against vibrating machines. Thus, on larger units, some technically desired sampling points are not practically monitorable and are virtually inaccessible.
To combat this variation on units where access is available, it is recommended that the same individual monitors each plate at each designated point on a particular vibrator unit at a sampling event. Due to this procedure, since the same individual can only monitor one location at a time, the sampling is temporally displaced for each monitoring point on respective plates. Even when the same individual performs the monitoring on a designated unit, the other variables listed above typically combine to create a great degree of subjectivity in the curves or plots generated. As such, many operators rely on specially trained vibrator unit technicians who periodically monitor the units for performance. Such technicians are trained to avoid the above-listed variables in card throw techniques; however while more accurate than average, their throw card data is still somewhat subjective. Also, as may be appreciated, there is limited availability of such technicians, who are also trained to diagnose the causes of substandard throw card curves or plots and their remedies.
Another drawback of the conventional throw card technique is that the monitoring is two-dimensional only, in the x and y-axes. Other than when a portion of a drawn curve is missing in one location or portion, indicating lateral motion of the unit, this conventional technique is incapable of accurately monitoring side-to-side (z-axis) movement of the unit. Such movement is an important indicator of plate asynchrony, due to the susceptibility of the plates to damage or accelerated wear caused by imbalanced forces acting in this direction. In view of the many causes for variation, it is estimated that as much as 70-80% of conventional throw card data is suspect.
In an effort to objectify the monitoring of plate movement using throw cards, some vibrator unit technicians have explored the use of accelerometers placed at desired unit monitoring locations. The accelerometers are positioned to monitor movement in the x and y-axes in similar locations to the placement of throw cards, and are electrically connected to a hand held device which plots appropriate curves. On a typical screen unit, pairs of accelerometers are placed at respective corners of the unit. Accelerometers have also been placed to monitor the bearing condition of the vibration generator. However, while this technique generates more objective data, conventional monitoring equipment has been designed to monitor data from one point on one plate at a time, and comparisons are typically restricted to a two-dimensional format (x and y-axes). While the implementation of accelerometers as discussed above shows promise in obtaining more objective and reliable vibrator unit performance data, the performance of vibrator screen units is very dynamic, and changes constantly with the volume and or type of material being screened. Thus, even the above-described technique involves inherent variability.
Thus, there is a need for an improved system for monitoring vibrator screen unit operation on a plate-by-plate basis to obtain more accurate data, even on larger vibrator units. There is also a need for an improved system for monitoring operational stresses on vibrator screen units which accounts for the dynamic nature of machine operation.